The invention is directed to an overload-protection circuit for use in switching-mode power converters. In particular, the invention is directed to a circuit for modulating the "off" time or the pulse repetition period of the switching element of the power converter, in response to an excessive switch current.
Overload protection is based on sensing the instantaneous current in the switching element (or, alternatively, in a filter inductor or energy-storage inductor which is coupled to the switching element), and on turning off the switching element as soon as possible if that current exceeds a chosen first threshold value. There are unavoidable delays in the circuits which perform the current-sensing, the comparison of the sensed current against the threshold, and the subsequent turn-off of the switching element. The current will continue to increase during that delay time. Thus, the peak value of the current will be somewhat higher than the first threshold value. The switch is next turned on at the next clock pulse for the case of a constant-frequency peak-current controller, or after a delay time set by an "off" pulse generator circuit for the case of a constant-"off"-time peak-current controller.
Short-circuit-current-runaway is the most serious hazard of overload-protection methods which use the constant-frequency or constant-"off"-time method of peak-current control. The runaway shows up in the output characteristic (output voltage plotted vs. output current) as a long tail of high output current as the converter output voltage collapses toward zero due to overload.
The physical reason for the short-circuit-runaway is that the minimum "on" time of the switch cannot decrease below the difference between the turn-off and turn-on delay times. This sets a minimum possible duty ratio D.sub.min for the converter, for any given switching frequency or any given "off" time.
The result of the current runaway is that the actual short-circuit current can be much larger than the value commanded by the first threshold signal. The difference between the actual value and the commanded value depends on the circuit parameters; in practical cases, it can be as much as an order of magnitude. That much difference can result in saturation of the magnetics, and overheating and eventual destruction of the power semiconductor devices in the power converter.
One known solution to the problem is to combine the constant-frequency or constant-"off"-time pulse-by-pulse protection with soft-start protection using PWM or peak-current-commanding control, as described by the inventors in "Overload Protection Methods for Switching-Mode DC/DC Converters: Classification, Analysis, and Improvements," PESC 87 Record, 18th Annual IEEE Power Electronics Specialists Conference, Blacksburg, VA, June 21-26, 1987 (IEEE Catalog No. 87CH2459-6), pp. 107-115. If the current exceeds the peak value commanded by the first threshold signal, but stays below a second threshold, current limiting is provided by the pulse-by-pulse peak-current-controlling method. If, however, the current exceeds the second threshold (a symptom of current runaway), the soft-start protection shuts down the converter and initiates a new soft start. With the PWM soft-start protection, this is an effective approach; it essentially eliminates most of the drawbacks of both the pulse-by-pulse and PWM soft-start schemes. The Signetics SE5560 PWM controller integrated circuit uses this approach.
However, with the peak-current-commanding soft-start protection, we cannot expect any improvement in the current-runaway phenomenon, because the circuit still suffers from the inherent delay between the turn-on of the switching element and the subsequent turn-off after detection of overcurrent. In that case, one must turn off the converter for a time long enough to allow the current in the filter inductor to drop sufficiently, and then initiate another soft start. It is inherently unsafe to depend on a peak-current-commanding soft-start scheme which programs the peak current to rise gradually at start-up. The absence of current runaway depends on a fortuitous combination of parasitic circuit parameters: inductor resistance; time delay of current sensor, controller, and switching element; and voltage drop in free-wheeling diode.
The object of the present invention is to provide protection from overcurrent under overload conditions, essentially free from the potentially destructive effects of current runaway described above.